The present invention relates to foundry tooling processes and methods and more particularly to a core assembly for use in casting operations.
Certain composite articles or castings have an insert or inlay integral with the casting. Examples of such inserts include valve seats for cylinder heads and steel or ceramic inserts for certain gray iron castings. The inserts may be snap fit or force fit into place to form the composite article. However, the present invention provides a method for forming such composite castings by using cast-in-place inserts. The composite castings thus formed not only provide a precisely positioned insert in the composite casting, but such inserts may be held in place by a metallurgical bonding of the insert to the cast metal.
To provide castings having inserts by the method of the present invention, it is important that the insert be precisely positioned in the casting, such as for the valve seat inserts mentioned above. When such precision is required, it is therefore critical that the insert be properly positioned when the casting is made. For such cast-in-place inserts, it is equally important that the location or positioning of the insert be maintained during the casting or molding process. Therefore, if the positioning of the insert is incorrect or if the location of the positioned insert changes during the casting or molding, casting defects and dimensional problems are likely to occur.
The present invention relates generally to a process in which a destructible plastic form is molded in the configuration of the article to be cast around a core. The composite plastic form and core is then placed in a mold cavity. Metal is cast into the mold cavity to vaporize the plastic form. The metal thus replaces the plastic form to form a shaped casting about the core which is thereafter removed.
It is known to use various methods of positioning articles in casting molds. For example, the "lost wax" process of casting, such as disclosed in U.S. Pat. Nos. 4,154,282 and 4,392,289, is used to cast a gem for the manufacture of jewelry. In the lost wax process, a gem is positioned in a mold and wax is cast therein to form a wax model; the model is subsequently covered with a non-destructible hardenable material, the wax is melted out, metal is cast into the void created by the wax and the hardenable material is removed. However, the lost wax process is different from the process employed in the present invention and presents different problems. The lost wax process involves introducing molten metal into voids created by the wax as opposed to the present process wherein the molten metal simultaneously vaporizes the plastic form and fills the space occupied by the plastic form. A problem associated with the lost wax process is that the article is supported by the hardenable material within the mold (i.e., the void space previously occupied by the wax), and it is therefore impossible to have an article supported within a void such that metal can be cast totally around the article because the lost wax process requires that the article be supported by the non-destructible hardenable material.
Another method for positioning an insert is by means of posts extending from the core box into the insert, as disclosed in U.S. Pat. No. 4,008,747. A disadvantage with this method is that the shape, orientation and location of the insert must be such that it can be supported by posts.
U.S. Pat. No. 4,243,093 discloses composite cast articles having insulating portions intermediate of the cast portions and which are contiguous with the outer configuration of the casting. This patent discloses forming destructible plastic portions which are assembled, such as with an adhesive, to be used in lost foam casting. There are many problems associated with the method of this patent. For example, the use of glue to join destructible portions means that the cores thus formed are subject to breakage due to handling. Excess glue may act as a core thereby creating a void in the casting. Similarly, to position the insulating article between the destructible portions, the entire assembly must be assembled step by step: forming the inner portions and gluing them together, positioning the insulating layer around the inner portions and forming the outer portions and gluing them together around the insulating layer. The present invention overcomes this burdensome task by forming a destructible pattern having an insert without the need to assemble the destructible portions about the insert.
It is, therefore, an object of the present invention to provide metal castings having inserts located therein. It is also an object of the present invention to provide a method for manufacturing such castings.
It is another object of the present invention to provide composite cores for the manufacture of metal castings having inserts therein and a method for manufacturing such composite cores.
It is a further object of the present invention to provide a method of manufacturing metal castings having inserts located precisely therein. It is also an object of the present invention to provide a method for the precise positioning of inserts in composite cores for the manufacture of such metal castings.